Bulletin 229, Diseases of Citrus in Florida


 CAUSE OF WITHERTIP
 Withertip and the accompanying injuries on citrus trees other
than limes are caused by the fungus Colletotrichum gloeosporioi-
des Penz. Strains of this organism, so similar that they are
not capable of being distinguished from one another, have been
found to have a
world-wide distribu-
tion. Work done in
California by Burger
showed that several
distinct strains of
this fungus exist in
the groves. One is
led to believe that
the amount of injury
to citrus trees and
fruits depends some-
what upon the viru-
lence of the strain of
the fungus as well as
upon the condition of
the tree.
 The fruiting
bodies of the wither-
tip fungus develop as
minute, dark, slight-
1 y raised, densely
massed pustules on Fig. 42.-Withertip russeting on grapefruit,
t h e twigs, leaves, caused by spores of the fungus on the dead
 twig above washing down over the fruit.
fruits, and dead bark
of citrus trees (Figs. 38 and 43). The fruiting bodies and spores
are often innumerable and may occur on almost any dead twig
throughout the grove. Spores of the fungus occur universally
on the surface of live twigs and leaves of healthy as well as dis-
eased trees.
 The withertip fungus, however, with the exception of the
species on lime, rarely attacks growing tissues unless the trees
are in a weakened or run-down condition. Trees weakened from
overbearing, drought, water injury, cold, lack of fertilization,
insect attacks, or almost any other cause are very susceptible
to the disease and it is on trees of this character that withertip